Happy with new organ (5)
I believe the answer is:
gland
'organ' is the definition.
(gland is a kind of organ)
'happy with new' is the wordplay.
'happy' becomes 'glad' ('glad' can be a synonym of 'happy').
'with' is an insertion indicator.
'new' becomes 'n'.
'glad' going around 'n' is 'GLAND'.
(Other definitions for gland that I've seen before include "Organ that secretes" , "it could be described as secretive" , "vital structure" , "Organ that releases essential substances into the bloodstream" , "Organ secreting chemical substances" .)